Abstract
Salivary gland polytene cells in the dipteran Chironomus
tentans provide exceptional experimental possibilities to
analyze processing of specific pre-mRNAs in intact eukaryotic
cell nuclei. Here we give a brief account of how these
experimental advantages can be exploited to analyze the splicing
process in vivo. In multi-intron pre-mRNAs, spliceosomes
assemble and splicing is initiated cotranscriptionally for all
introns. Intron excision may, however, occur mainly
cotranscriptionally or mainly posttranscriptionally depending on
the position of each intron in relation to the remaining
transcription time and intron-specific efficiencies of excision.
As measured for the U2 snRNP and an SR protein, 10-15% of the
spliceosomal components are bound to pre-mRNA at active gene loci
at a given moment, while the majority of the spliceosomal
components are present in the nucleoplasm. A continuous
redistribution of the spliceosomal components takes place in the
nucleus as a result of a close coupling between transcription and
spliceosomal assembly.